Electrochemical cell with platinum-ruthenium electrode and method of using with ammonia

ABSTRACT

An improved method of, and an apparatus for, generating electrical energy is described comprising feeding ammonia to an anode comprising a major amount of platinum and a second metal of the platinum group in minor but effective amounts. Preferred percentages range from about 75 to 98 parts platinum and from about 25 to about 2 parts of the promoter metal on an atomic weight basis.

United States Patent Giner et al.

[ Mar. 21, 1972 [541 ELECTROCHEMICAL CELL WITH 3,305,402 2/1967 Jones etal. ..136/120 PLATINUM-RUTHENIUM ELECTRODE 3,306,780 2/1967 Dieberg..136/121 AND METHOD OF USING WITH 3,309,231 3/1967 Hess ..136/120AMMONIA FOREIGN PATENTS OR APPLICATIONS [721 Inventory Jose Gin",Glastonbury; James 951,168 3/1964 Great Britain ..136/86 Moser, Bolton,both of Conn.

[73] Assignee: United Aircraft Corporation, East Hart- OTHERPUBLICATIONS ford, C(mn- Young Fuel Cells-Vol. Z-Reinhold Publ. Co., NY.1963, pp. 153, 155, 156, 160, 161 162, 164- Article by R. A. Wyn- F l[22] Sept 965 veen- Preliminary Appraisal of the Ammonia Fuel Cell [21]Appl.No.: 491,759 System-ChapterlZ W. A. Nemilow et al. UeberLegierungen des Platins mit [52] U.S. c1 ..136/86, 136/120 z f g g i g VChem- 22 [51] Int. Cl ..Hlm 27/10, HOlm 27/30 [58] Field of Search..136/86 E, 86 D, 120, 120 PC; Primary Examiner winston A. DouglasAssistant Exammer-M. J. Andrews 56] References Cited Attorney-Robert F.Conrad and Alfred W. Breiner UNITED STATES PATENTS 57] ABSTRACT3,321,334 /1967 Palmer ..136/86 An improved method of, and an apparatusfor, generating 3,368,922 2/1968 Salyer..... ...l36/86 electrical energyis described comprising feeding ammonia to 3,332,103 5/1963 Smith a --lan anode comprising a major amount of platinum and a 3,431,220 1969Batlold 2/4 1 X second metal of the platinum group in minor buteffective 2,076,953 4/1937 y /172 UX amounts. Preferred percentagesrange from about 75 to 98 2,636,819 4/1953 StreIcher.... ..75/172 partsm g and f about 25 to about 2 parts f the COhn et a]. 1 promoter metalon an atomic basis 3,288,653 11/1966 Holteta1.... ....l36/l20 3,297,4891/1967 Feng et al. ..136/ 3 Claims, 3 Drawing Figures l'LO ACTlVlTY u/rn co 1 50 100 o Ru (Remainder a c.% l.-.\

' idation 'of ammonia.

A fuel cell," as the term is generally understood in the art, refers toa device which converts chemical energy directly into electrical energywherein the over-all cell reaction is the oxidation of 'a fuel by oxygenor suitable oxidizing gas, such as air. The essential components of sucha fuel cell are two electrodes in contact with the oxygen-containing gasand the fuel, respectively, and an electrolyte. In accordance withgenerally recognized convention, the oxygen electrode may be consideredas the positive electrode and the fuel electrode as the negativeelectrode with reference to the external circuit. The electrolytefunctions to permit transport of ions without direct electrical contactbetween the fuel and oxidizing gas whereby the oxidation of the fuel cantake place only as a result of a directed flow of ions across theelectrolyte and a corresponding flow of electrons in an externalcircuit.

The chemical and physical characteristics of ammonia make itcommercially attractive as. a fuel because of its low cost and readyliquification. Its liquid state permits ease of transportation, and morecritically, the use of low cost equipment for the transportation andstorage of the material. Ammonia has the additional property of beingreadily soluble in an aqueous electrolyte. Therefore, with a highconcentration of fuel, diffusion is not likely to cause appreciablepolarization at the electrode. Thus, the rate of the electrochemicalreaction at the electrode will be the principal limiting factoraffecting the current density of the fuel cell. However, heretoforeavailable electrodes employing metals of the platinum group such asplatinum, ruthenium, iridium, and the like, have demonstrated only verylow catalytic activities in an aqueous electrolyte when fed with ammoniawith resultant low current densities being obtained. Such low currentdensities precluded the use of ammonia as a fuel in a practical cell.

It is, therefore, an object of the present invention to provide anelectrode for a moderate temperature fuel cell, for theelectro-oxidation of ammonia, which electrode develops remarkablyenhanced current densities at low polarization rates.

For further objects and advantages of the invention, for a descriptionof the methods of producing the preferred form of the electrode of thepresent invention, and for an outline of the ways of practicing theinvention, reference is to be had to thefollowing description taken inconjunction with the drawing illustrating test results of electrodesaccording to the invention.

The present invention is particularly concerned with the production of afuel cell electrode for the electro-oxidation of ammonia to producedirect current energy, whichrepresents'a different order ofeffectiveness and which provides an unexpectedly low'propensity towardpolarization of the voltage for a given current output. The electrode ofthe present invention is designed primarily for use in an improved fuelcell constituted to utilize ammonia as the fuel in anaqueouselectrolyte, particularly an aqueous alkali metal hydroxide, at moderatetemperatures.

More particularly, the fuel cell electrode of the present invention ischaracterized by the presence of a catalytic alloy consistingessentially of platinum and a second metal of the platinum group, e.g.,ruthenium or rhodium, in which the platinum is present in a major amountand the second metal of theplatinum group is present in a minor amount.The activity of these electrodes for the electro-oxidation of aqueousammonia is substantially greater than the activity of either' platinumor the second metal of the platinum group taken individually. Morespecifically, the platinum and second metal of the platinum group arepresent in an atom ratio of from about 75 to about 98 platinum and fromabout to about 2 of the second metal of the platinum group. As anexample,

platinum-ruthenium and platinum-rhodium alloys within the aforesaidrange, bonded to a compatible electrode substrate, exhibit enhancedcatalytic electrochemical oxidation of ammonia at low rates ofpolarization of the electrode voltage when in simultaneous contact withan aqueous caustic electrolyte. Although the above ratios are effective,the preferred catalytic alloys consist essentially of platinum andruthenium in an atom ratio of from about 75 to about platinum and fromabout 25 to about 15 ruthenium and platinum and rhodium in an atom ratioof from about 92 to about 98 platinum and from 8 to about 2 rhodium,preferably as the black forms.

The catalytic alloys can be formed upon a suitable substrate to providethe electrodes of the present invention by any appropriate method suchas vapor deposition, chemical deposition, electro-deposition and thelike which provides the requisite electrolyte-catalyst interface for theelectrochemical oxidation reaction to proceed at optimum rates. Apreferred manner of producing the electrodes of the present invention,however, is the electrodeposition of the black forms of the respectivemetals from an aqueous solution of a mixture of their soluble salts,e.g., the chlorides thereof, at suitable voltage and current densitiesto form a stable bond between the catalytic alloy and the substrate.Thus, a suitable substrate can comprise a porous matrix formed bysintering other metal powders, such as copper, nickel, iron, platinum,and the like,

thereby forming a porous structure, and thereafter coating at least thesurface of the pores which comes in contact with the aqueouselectrolyte, with the desired platinum alloy. Furthermore, a porousmatrix comprising a porous refractory of ceramic or polymeric materialwhich is then coated with the desired platinum alloy can also beutilized. Accordingly, the substrate can be an electroconductor ornon-conductor. Where the electrode substrate itself is notelectro-conducting, the thickness of the alloy deposited should besufficient to conduct current and provide connecting means to theexternal conducting circuit, or a conductive screen or mesh can bepressed into the catalytic layer. The electrodes can be fabricated asflat, unsupported sheets or they may be formed as a corrugated ortubular structure. The tubular construction is sometimes preferred sincethe effective surface area of the electrode is increased and is idealfor bi-polar or multi-polar cells. Additionally, a tubular structurewill withstand greater pressures.

Electrodes made in accordance with the invention were experimentallyevaluated as fuel electrodes for the electrochemical oxidation ofammonia at various temperatures. In order to eliminate uncontrolledvariables which conceivably could mask the actual performance obtainedfrom the fuel electrode, a half-cell electrode test unit was employed.The fuel electrode to be tested was opposed by a platinum cathode, bothimmersed in 5N aqueous KOH electrolyte and an external source of currentwas supplied to the platinum cathode. This measured current,representing the total electrode current, is then related to theoxidation potential of the fuel electrode in reference to a reversiblehydrogen electrode in the same electrolyte which is used as a referenceelectrode.

As exemplary of the improved results obtainable from the use ofelectrodes employing the platinum-ruthenium alloys as catalysts,reference is had to FIG. 1. This figure shows the test results at 25 C.of a number of catalytic alloys prepared by electrodeposition of theblack forms of the metals onto a platinum substrate from an aqueoussolution of their chlorides having a total concentration of platinumplus ruthenium equal to 2 weight percent at ambient temperature (25 30C.) at from about 3 to about 4.5 volts and at a current ranging fromabout 75 to milliamps per square centimeter. In the term Activity,,uA/mcoul of FIG. 1, uA refers to the ammonia current obtainable at 400mv positive to a hydrogen electrode in the same solution and mcoulrefers to the quantity of electricity necessary to form a monolayer ofhydrogen on the surface of the catalyst, i.e., a factor describing thequantity of available surface. As the atom ratio of ruthenium'platinumin the plating solution increases, the activity of the deposited alloyincreases rapidly, reaching a maximum at a composition of approximately20: 5/80: 5 ruthenium-platinum in the plating solution. Alloys depositedfrom plating solutions having an atom ratio of ruthenium to platinum offrom about 5/95 through 50/50 demonstrate a significant enhancement ofthe catalytic activity. Alloys deposited from plating solutions havingan atom ratio of ruthenium to platinum of from about /90 to about 25/75have greatly enhanced catalytic activity.

As exemplary of the improved results obtainable from the use ofelectrodes employing the platinum-rhodium alloys as catalyst, referenceis had to FIG. 2 showing test results at 25 C. of catalytic alloysprepared by electrodeposition of the black forms of the metals on aplatinum substrate as described hereinbefore in reference toplatinum-ruthenium alloys. As the atomic percent rhodium in the platingsolution increases, the activity of the deposited alloy initiallyincreases rapidly, reaching an optimum when the atom ratio ofrhodiumplatinum is near 5/95 (Le, 5:3 rhodium and 95:3 platinum) in theplating solution. Substantial activity is demonstrated by alloysdeposited from plating solutions having an atom ratio ofrhodium-platinum offrom about 2/98 to 25/75.

Additional experimental data indicative of the improved performance ofthe electrodes tested are set forth in the potential vs. current curvesof HO. 3. The tests were carried out at 45 C. in 5 molar KOH with theelectrolyte being saturated with ammonia. In the potentiostaticexperiment, the iR loss is eliminated. The composition of the electrodesin atomic percent is as follows:

From the curves, it is seen that the performance of the alloys ofplatinum and ruthenium, and platinum and rhodium are greatly superior tothe pure platinum, pure ruthenium, and pure rhodium electrodes. Thesuperiority is primarily in lower polarization values and greaterlimiting current densities.

The electrodes described herein can be employed in the generation. ofelectrical current by the electro-oxidation of ammonia in substantiallyany of the prior art methods of feeding the reactants. Thus, the ammoniacan be brought into solution with the electrolyte and thereafter intocontact with the electrode, or it can be fed directly to the electrodein the gaseous or liquid state. Moreover, the electrolyte, whilepreferably being an alkali metal hydroxide, can be virtually any of theknown prior art electrolytes. It is only necessary that the electrolyteremain substantially invariant and have high ionic conductivity at thereaction conditions of the fuel cell. Therefore, other alkalineelectrolytes such as the carbonates and the alkanolamines can beemployed. Further, although it is indicated that the operatingtemperature of the cell is preferably below 100 C., it is, of course,possible to operate the cell at higher temperatures. Thus, fuel cellsemploying the electrodes of this invention will function satisfactorilywithout loss of integrity at temperatures as high as 450 to 500 C.Further, although the presently described electrodes are primarilydesigned for use in conjunction with ammonia as the fuel, they dopossess enhanced properties for other fuels including hydrogen, carbonmonoxide, methane, methanol, propane, and the like. The most outstandingresults, however, are realized in the electro-oxidation ofammonia.

In the fuel cells utilizing the electrodes of the present invention, anyof the prior art cathodes can be employed such as the lithiated nickeloxide structures as described in Bacon, U.S. Pat. No. 2,716,670. Inthese fuel cells, the ancillary hardware is not of major importance. Itis only necessary that the fuel cell comprise the anode of the presentinvention, a cathode as described in the prior art, with an electrolytetherebetween, and means for feeding the fuel and oxidant to therespective electrodes. The spacial arrangement of the components of thecell are not critical. However, it is usually desirable to have the cellas compact as possible in order to utilize as little space as possible.Additionally, the electrodes should be closely spaced in order that theohmic resistance across the electrolyte is as small as possible. As willbe apparent to one skilled in the art, it is possible to make numerousmodifications to the invention described without departing from theinventive concept herein disclosed. Such embodiments being within theability of one skilled in the art are to be embraced by the followingclaims.

What is claimed is:

1. A fuel cell for the direct generation of electricity from ammonia andoxygen comprising: an oxygen electrode as the cathode; an ammoniaelectrode as the anode; an electrolyte therebetween; said electrodesbeing spaced apart and in contact with said electrolyte, said anodecomprising a platinumruthenium alloy catalyst for promoting theelectro-oxidation of ammonia, the atom ratio of platinum to ruthenium insaid alloy catalyst being from about 75 to about platinum and from about25 to about 15 ruthenium; a source of oxygen including means forproviding oxygen to said cathode; and a source of ammonia includingmeans for providing ammonia to said anode.

2. A fuel cell according to claim 1 wherein the electrolyte is anaqueous alkali metal hydroxide solution.

3. A method for the direct generation of electricity utilizing a fuelcell which comprises an ammonia anode, an oxygen cathode, andelectrolyte therebetween, comprising the steps: bringing ammonia intosimultaneous contact with said electrolyte and said anode; said anodecomprising a platinumruthenium alloy catalyst for promoting theelectro-oxidation of ammonia, the atom ratio of platinum to ruthenium insaid alloy being from about 75 to about 85 platinum and from about 25 toabout 50 ruthenium; simultaneously bring oxygen into contact with saidelectrolyte and said cathode; and connecting said anode catalyst andsaid cathode by means of an electrical conducting circuit which receivesthe generated electricity.

2. A fuel cell according to claim 1 wherein the electrolyte is anaqueous alkali metal hydroxide solution.
 3. A method for the directgeneration of electricity utilizing a fuel cell which comprises anammonia anode, an oxygen cathode, and electrolyte therebetween,comprising the steps: bringing ammonia into simultaneous contact withsaid electrolyte and said anode; said anode comprising aplatinum-ruthenium alloy catalyst for promoting the electro-oxidation ofammonia, the atom ratio of platinum to ruthenium in said alloy beingfrom about 75 to about 85 platinum and from about 25 to about 50ruthenium; simultaneously bring oxygen into contact with saidelectrolyte and said cathode; and connecting said anode catalyst andsaid cathode by means of an electrical conducting circuit which receivesthe generated electricity.